Yes they are pretty good. However the biggest problem you have is that you will not get anywhere near one for £3,500!
You may well get a Focus TD or TDDI. However these engines are not very good at all, and you'd be better off with a 1.6 Petrol focus.
If you're budget is £3,500, then a good diesel motor to look at is a Seat Toledo(/Leon). For your money, you should get a ~2000 1.9TDI 110 SE model.
Or you could look at something Peugeot/Citroen with a HDI engine in it.
There's been lots of threads recently about the pro's and con's of diesel vs. petrol and the economics of it, rather than dredge up the topic again have a search and see what you think, and how it applies to you.
The argument is basically; You have to spend more ££ to get a decent diesel compared to a petrol model, and you need to do a lot (20k+) of miles to make the in price difference up.
For his budget get can get a 306 HDI, on about a V/X/W plate.